Consistently use only one of virtual/OVERRIDE/FINAL.

build/standalone.gypi

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 220 matching lines @@
         ],
         'ldflags': [
           '-fsanitize=thread',
           '-pie',
         ],
         'defines': [
           'THREAD_SANITIZER',
         ],
       },
     }],
-    ['clang==1', {
-      'target_defaults': {
-        # Remove once issue 3753 is fixed.
-        'cflags_cc': [ '-Wno-inconsistent-missing-override',
-                       '-Wno-unknown-warning-option', ],
-       },
-    }],
     ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris" \
        or OS=="netbsd"', {
       'target_defaults': {
         'cflags': [
           '-Wall',
           '<(werror)',
           '-W',
           '-Wno-unused-parameter',
           '-Wno-long-long',
           '-pthread',
@@ skipping 199 matching lines @@
       ],
     }],
     ['clang==1 and OS=="win"', {
       'make_global_settings': [
         # On Windows, gyp's ninja generator only looks at CC.
         ['CC', '<(make_clang_dir)/bin/clang-cl'],
       ],
     }],
   ],
 }